The field of the invention is systems and methods for magnetic resonance imaging (“MRI”). More particularly, the invention relates to systems and methods for imaging magnetic particles with MRI.
In recent years, a method has been proposed for administering magnetic particles, such as super paramagnetic iron oxide (“SPIO”), that serve as a contrast medium into a subject and forming an image of a distribution of the contrast medium. Such methods are referred to as magnetic particle imaging (“MPI”). MPI differs from conventional MRI in many ways. First and foremost, MPI relies on a unique imaging system that is different than conventional MRI systems. Also, in MPI, an image of the distribution of magnetic particles within a subject is formed by measuring a change in the voltage induced in a detection coil as a driving magnetic field magnetizes the particles in a non-linear fashion.
In MRI, magnetic contrast agents are generally visualized by way of their effect on the relaxation times T1, T2, and T2*. For example, magnetic contrast agents may shorten the T1 of those nuclear spins near to the contrast agent, thereby altering the signal detected from those nuclear spins and producing a contrast mechanism.
A method for modulating the contrast achieved by an MRI contrast agent referred to as “dreMR” is taught by J. K. Alford, et al., in “Delta Relaxation Enhanced MR: Improving Activation-Specificity of Molecular Probes through R1 Dispersion Imaging,” Magnetic Resonance in Medicine, 2009; 61(4):796-802. The dreMR method achieves this contrast modulation because for rapidly tumbling paramagnetic agents the longitudinal relaxation time, T1, which is the reciprocal of the longitudinal relaxation rate, R1, varies little over the range 1.2-1.8 Tesla (“T”), while slowly tumbling, bound species elicit a steep dependence of T1 relaxation over the same field range. Using the dreMR method, comparisons of images undergoing relaxation at different field strengths allows direct separation of the relaxation effects of the free versus the bound pools as well as allowing subtraction of the two images. In dreMR, both the background tissue and the free contrast agent pool are subtracted out. While clinical MRI systems have a fixed field strength, such as 1.5 T, it is feasible to build and operate an insert compatible with clinical MRI systems that can produce enough change in the B0 field to significantly modulate the relaxation of bound contrast agents.
In light of the foregoing, it would be advantageous to provide a system and method for acquiring contrast-enhanced images of a subject that has been administered a magnetic particle contrast agent. Such a system and method would preferably utilize a conventional, clinical MRI system, rather than require a unique imaging system (as required for MPI) or a significant modification to a standard MRI system (as required for dreMR). The benefits of being able to utilize a clinical MRI system include improving MPI by allowing spatial encoding to be done by MRI rather than inductive pick-up or altering an external static magnetic field, thereby improving spatial resolution and imaging quality than current MPI. It would also be beneficial to image a contrast agent by a means other than the contrast agent's effect on the conventional T1, T1ρ, T2, or T2* relaxation characteristics because natural variations in these characteristics in the subject may need to be distinguished from the effect of the contrast agent.